Calculating Tension on a Ramp with Inclined Load and Friction

[songoku]

Homework Statement

A sack of weight 600 N is being loaded on to a truck up a ramp inclined at 18^o to the horizontal. A rope attached to the sack is held at an angle of 25^o to the ramp. The coefficient of friction between the sack and the ramp is 0.3. What tension in the rope is needed:
a. To prevent the sack from sliding down the ramp.
b. To pull the sack up the ramp at a steady speed

Homework Equations

F = ma

The Attempt at a Solution

The manual wrote the answer for (a) is 18.2 N. I can't find it. My work for (a) :

W sin 18^o = T cos 25^o

T = 204.6 N

What am I missing ?

Thanks

 

[rl.bhat]

You have not taken into account the frictional force while calculating T.

 

[songoku]

Hi rl.bhat

Oh my god...I thought there is no friction because it didn't move. That was a very stupid mistake. Thanks a lot !

 

[radou]

I thought there is no friction because it didn't move.

Of course there is - it's called static friction.